Scientists say they’ve discovered the largest recurring algae bloom in the world, and it stretches from the coast of West Africa to the Gulf of Mexico with no signs that the “new normal” pattern will stop anytime soon.

The research team, led by scientists at the University of South Florida (USF) College of Marine Science, used NASA satellite data to zoom in on the brown macroalgae called Sargassum to better understand its behavior. What they found was that it can blanket the surface from one continent to the other, and when it did so in 2018 it weighed more than 20 million tons – a weight heavier than 200 fully loaded aircraft carriers. They published their research results last week in the journal Science.

The Sargassum is welcome in smaller doses on the open ocean, where it supports fish and other marine life, and boosts ocean health by producing oxygen through photosynthesis. Mariners on the Atlantic Ocean have noted its presence since the 15^th century, but too much of it creates problems, especially when it proliferates and forms thick mat structures in coastal areas.

That’s been the case since 2011 when an uptick in coverage was first noted, and now it’s spanning the 8,850-kilometer distance between coasts. Sargassum started to flourish in places it hadn’t been before, and came in vast amounts that smothered the coasts and created problems for local governments.

“Some countries, such as Barbados, declared a national emergency in 2018 because of the toll this once-healthy seaweed took on tourism,” said NASA.

Too much Sargassum makes it harder for some marine species to breathe and it decreases their mobility. When the algae dies, it sinks to the ocean floor and can choke out corals and seagrasses. It’s also a problem for people.

“On the beach, rotten Sargassum releases hydrogen sulfide gas and smells like rotten eggs, potentially presenting health challenges for people on beaches who have asthma, for example,” said the research team. Yet if it’s people who are also threatened, it is people who are responsible in part for the problem.

The scientists think what’s happening has two causes. Part of the year, Amazon River discharges dump nutrients into the ocean and these may have increased in recent years because of deforestation and agricultural fertilizer use. In the winter, off the West African coast, upwelling currents deliver nutrients to the surface where Sargassum can grow.

The team analyzed fertilizer consumption patterns in Brazil, alongside Amazon deforestation rates, Amazon River discharge, two years of nitrogen and phosphorus measurements taken from the central western parts of the Atlantic Ocean, and other ocean properties. More study is needed, but there’s a clear pattern that links the blooms to agricultural runoff.

Viewed through a wider lens, the algae bloom is evidence that something has changed in the ocean’s biochemistry to support Sargassum populations that have gone wildly out of control, said USF scientist Chuanmin Hu, who has studied the algae since 2006.

“This is all ultimately related to climate change because it affects precipitation and ocean circulation and even human activities, but what we’ve shown is that these blooms do not occur because of increased water temperature,” Hu said. “They are probably here to stay.”

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